Australia ended combat operations in Afghanistan and withdrew about 1000 troops in 2013 but still has about 400 troops performing advisory and training jobs.

But the presence of any international forces beyond 2014, when the International Security Assistance Force mission officially concludes, remains uncertain because of the absence of an agreement with the Afghan government.

The US and Afghanistan have negotiated a bilateral security agreement, but Afghan President Hamid Karzai refused to sign, saying that should be done by his successor.

Both the US and Australia had threatened to withdraw all forces if agreement could not be reached soon.

The new president is tipped to be Abdullah Abdullah, who is the likely victor of the run-off poll to be held on June 14.

Senator Johnston said ongoing involvement of the US, Australia and other Western nations was contingent on Afghan agreement of the Bilateral Security Agreement and a Status of Forces Agreement with NATO.

Appropriate legal arrangements would be the key to any future Australian mission.

"NATO and partners are at an important point in the planning for a Status of Forces Agreement with Afghanistan in order to provide the legal framework for the post-2014 mission," he said.